1. Field of Invention
The present invention is directed to core-shell types of metal, metal oxide and alloy nanoparticles with variable core diameters and shell thicknesses, and controlled synthesis methods for producing such nanoparticles. The present invention is also directed to nanocomposite materials fabricated from nanoparticles, such as the nanoparticles provided by this invention, and methods for producing such nanocomposite materials.
2. Description of Related Art
Nanoparticle based materials have been under investigation because of their novel catalytic, electronic, magnetic and optical properties. Among the various materials, transition metal nanoalloys containing core-shell types of nanoparticles are particularly interesting. The magnetic and chemical properties of these nanoparticles have significantly enhanced functions that do not exist in single-component compounds combined with unique properties that exist only in nm-sized materials.
Klabunde et al., J. Chem. Mater., 14: 1806–1811 (2002), demonstrated that Fe2O3-coated metal oxide core-shell nanoparticles such as MgO@Fe2O3 and CaO@Fe2O3 have greatly enhanced efficiencies over pure MgO and CaO catalysts for SO2 adsorption, H2S removal, and chlorocarbon destruction. The improved catalytic behavior comes from the cooperative interaction between the core and the shell of the catalyst.
Luo et al., Catal. Today, 77: 127–138 (2002), showed that gold-based core-shell nanoparticles can efficiently catalyze the electrooxidation of methanol. The metal-oxide-on-metal core-shell configuration (Au@AuOx) is believed to be the contributing factor for the high catalytic behaviors. Chen et al., J. Catal., 205: 259–265 (2002), showed that well balanced bimetallic surfaces of nickel on platinum can drastically enhance the catalytic hydrodesulfurization of thiophene per surface metal atom over a pure platinum surface.
Recent efforts have focused on the development of magnetic nanoalloys for their potential use for ultrahigh density magnetic recording media. Nanoalloy particles offer the advantages of significantly higher magnetic anisotropy, enhanced magnetic susceptibility, large coercivities and good chemical corrosion resistance. Transition metal alloy nanoparticles such as CoPt and FePt are excellent candidates.
Previous synthesis methods of magnetic alloys include vacuum deposition, metal evaporation, sintering and co-sputtering. These techniques allow limited control over particle size, size distribution and composition distribution in the nanocomposite materials.
More current synthesis methods include chemical approaches in solution and offer better control of magnetic nanomaterial growth. These techniques are largely based on the reduction of metal salts by a borohydride or by a diol or polyalcohol (the “polyol process”), and the thermal decomposition of organometallic precursors. For instance, U.S. Pat. No. 6,302,940 B2 to Murray et al. describes the synthesis of magnetic FePt alloy nanoparticles by the combination of in-situ reduction of platinum acetylactonate (CH3COCHCOCH3 anion) by long chain diol and thermal decomposition of Fe(CO)5 in the presence of mixed surfactants. Using a similar method, Murray et al. further describe, in MRS Bulletin 26, 985–991 (2001), the synthesis of CoNi alloy nanoparticles using Co2(CO)8.
The polyol process creates monodisperse FePt nanoparticles in disordered face-centered cubic (fcc) phases at low temperatures. The assembly of FePt particles can then be converted into face-centered tetragonal (fct) FePt hard magnetic crystalline materials by annealing at temperatures of about 650° C. However, using the approach developed by Sun, Murray et al., the nanostructures are lost after the annealing process The formation of monodisperse FePt nanoparticles with different sizes are believed to be governed by the surfactants that chemically bond to the surfaces of FePt nanoparticles.
Nanocomposite is another extremely valuable class of hard-magnetic materials. Theoretical predictions suggest that composites with nanometer sized soft and hard magnetic domains (<about 10 nm in diameter), created through a so-called exchange coupling mechanism, can possess a high energy product that does not exist in single phase materials.
The availability of such nanocomposites, however, are very limited because it is not trivial to precisely control the nm scale domains for both soft and hard magnetic materials simultaneously in magnetic composites. Moreover, the tolerances for the performance of modern materials and devices based on such magnetic materials requires high uniformity.
Zeng et al., Nature, 420: 395–398 (2002), recently demonstrated a method for making a magnetic nanocomposite film with interwoven structures of hard-soft magnetic domain using FePt and Fe3O4 nanoparticle assembly. In this bottom-up approach, FePt and Fe3O4 nanoparticles were allowed to self-assemble and subsequently were converted to FePt—Fe3Pt alloy thin films at enhanced temperatures. The exchange coupling between soft and hard components (FePt and Fe3Pt) produced a mixed hard-soft nanocomposite with much higher magnetic energy product than pure FePt alloy.
One apparent difficulty in this approach that could limit the energy product is the homogenous mixing of two entirely different monodisperse magnetic nanoparticles of Fe3O4 and FePt. Accordingly, homogeneity in the final product is achieved only in small quantities in local regions. Producing fine regular nm-sized domains over a large area, important for efficient exchange coupling, is crucial. The unmediated self-assembly of different types of nanoparticles can only create inadequate packing orders with almost no control of the packing density and number of layers deposited.
As demonstrated by Skumryev et al, Nature, 423: 850–853 (2003), in addition to exchange-coupled nanocomposites, exchanged-bias nanocomposites are yet another class of important magnetic materials that hold promise for high magnetic storage media applications.